Public meeting:
In Public meeting anybody can join your meeting if they know your meeting room name.

The host can send an email or IM message to invited guests with the following URL where "roomname" is simply the user's DimdimID

http://webmeeting.dimdim.com:80/portal/JoinForm.action?confKey=roomname

Anytime the host is in a live meeting, anyone can join by via this URL or by going to Dimdim.com, clicking the "Join meeting" button and entering the meeting room name.

Private meeting:
In private meeting an attendee can join only if they knows both the meeting room name and the meeting key. The room name will be the same but the meeting key can be different for each meeting. While starting a meeting host can specify the unique meeting key and send it separately from the meeting invitation.  If an uninvited guest goes to Dimdim.com, clicks the "Join meeting" button and enters your meeting room name while you are in a live private meeting, they will be prompted to enter your meeting key.